1. Field of the Invention
The present invention relates generally to analog active filters, and more particularly, pertains to integrable analog active filters utilizing only integrating amplifiers and ratioed capacitors.
2. Description of the Prior Art
Recently, there has been considerable interest in the development of active filters suitable for fully integrated monolithic implementation. Standard active RC filter design has not been normally useful for full integration because of the need for capacitors which are too large for integrated circuit technology.
Active R filters have received attention because they use integrating amplifiers, no capacitors, and establish filter parameters via ratios of resistors. Active R filters employ capacitors for only internal amplifier compensation. In active R filters, amplifiers are used as integrators and filter parameters such as pole frequency, quality factor, and gain are established with resistor ratios. Active R filters have the advantage of being suitable for operation over wide ranges of frequency, audio to video, but are prone to drifts, and use relatively large amounts of silicon area. Although the active R filters are easy to realize in integrated circuit technology, resistor ratios are less advantageous than ratios of capacitors for integrated circuit implementation, especially in MOS form.
Sampled-data analog switched-capacitor filters in which resistors are simulated via switched capacitors have been shown to perform very well at low frequencies but operation at high frequencies is far less than satisfactory because of problems associated with the need for very high switching frequency to avoid aliasing, with switching noise, and with the finite charging and discharging time of the capacitors. At the high frequencies, the internal amplifier dynamics introduce additional poles and phase shifts giving rise in switched-capacitor filters to the same problems encountered in active RC filters at the high frequencies.
With few exceptions, none of them useful for MOS implementation, filters which have been fully implemented in integrated form to date are either digital in nature or belong to the class of sampled data filters, such as switched-capacitor circuits. None of the prior art filters are readily integrable in MOS technology, and, thus, lack compatibility with modern integrated digital signal processing systems.
The integrable analog active filter proposed in this invention avoids most of the problems of the prior art filters.